Masking layer in substrate cavity

ABSTRACT

A package that resists creation of particles in a package cavity. The package is ideally suited for use with microelectromechanical devices. A package according to one embodiment of the present invention contains a mechanical device ( 302 ), for example a micromirror device, attached to the floor ( 304 ) of the package substrate ( 306 ). Epoxy ( 308 ) typically is used to attach the device ( 302 ). Electrical connections are provided by bond wires ( 310 ) connecting bond pads ( 312 ) on the substrate ( 306 ) with bond pads on the device ( 302 ). A window ( 314 ) is attached to the substrate ( 306 ) to form a cavity ( 316 ) around the device. A thin masking layer ( 318 ) on portions of the package cavity surface prevents the surface from generating particles. The thin masking layer ( 318 ) may be any material that resists particle generation. Malleable metals are ideal. For example, gold plating on the inner cavity walls bonds well to the ceramic surfaces, captures particles that are already loose, and resists chipping to avoid the generation of additional particles. The masking layer ( 318 ) on the cavity walls optionally extends out of the cavity and onto the upper surface of the package. The preceding abstract is submitted with the understanding that it only will be used to assist in determining, from a cursory inspection, the nature and gist of the technical disclosure as described in 37 C.F.R. §1.72(b). In no case should this abstract be used for interpreting the scope of any patent claims.

FIELD OF THE INVENTION

[0001] This invention relates to the field of semiconductor packaging,more particularly to packaging of micro-electro-optical systems (MEOMS)and micro-electro-mechanical systems (MEMS).

BACKGROUND OF THE INVENTION

[0002] Micromechanical devices are small structures typically fabricatedon a semiconductor wafer using techniques such as optical lithography,doping, metal sputtering, oxide deposition, and plasma etching whichhave been developed for the fabrication of integrated circuits.

[0003] Micromirror devices are a type of micromechanical device.Micromirror devices are primarily used in optical display systems. Indisplay systems, the micromirror is a light modulator that uses digitalimage data to modulate a beam of light by selectively reflectingportions of the beam of light to a display screen. Other types ofmicromechanical devices include accelerometers, pressure and flowsensors, gears and motors. While some micromechanical devices, such aspressure sensors, flow sensors, and micromirrors have found commercialsuccess, other types have not yet been commercially viable.

[0004] Packaging MEOMS and MEMS presents special challenges. In the caseof the micromirror device, the package must provide a hermetically, ornear-hermetically sealed environment while providing a distortion-freeoptical path to the surface of the device and a large number ofelectrical connections to the device. The package must also me robustenough to withstand extreme thermal gradients when the micromirror arrayis illuminated by a high optical flux beam.

[0005] Furthermore, the package must not create particles during thelife of the device. Particles generated within the device can enter themoving superstructure of the MEMS device and electrically short circuitportions of the device or mechanically block the movement of the movingparts. Because of the moving parts and the requirement for some devicesto maintain an optical path with the device in the package, the packagecavity cannot simply be filled with a potting compound.

[0006] Ceramic substrates are preferred for micromirror devices becausethey provide a good barrier against moisture entering the package.Additionally, ceramic substrates are mechanically robust, allowing themto withstand stresses generated by uneven heating that occurs when themicromirror is illuminated. Unfortunately, ceramic substrates generateparticles that can destroy the packaged devices. What is needed is amethod and system for preventing particle generation by ceramic packagesubstrates.

SUMMARY OF THE INVENTION

[0007] Objects and advantages will be obvious, and will in part appearhereinafter and will be accomplished by the present invention whichprovides a method and system for coating the inside of a ceramic packagesubstrate to prevent particle generation. One embodiment of the claimedinvention provides a method of forming a package substrate. The methodcomprising: forming a substrate, the substrate having a cavity; andcoating portions of the cavity surface with a masking layer.

[0008] Another embodiment of the present invention provides a package.The package comprising: a substrate having an upper surface and a cavitysurface formed therein, the cavity having at least one wall surface andat least one floor surface; a device attached to the cavity floor; a lidattached to the upper surface enclosing the device; and a masking layeron at least a portion of the cavity surface.

[0009] Yet another embodiment of the present invention provides apackage substrate. The package substrate comprising: a substrate havingan upper surface and a cavity surface formed therein, the cavity havingat least one wall surface and at least one floor surface; and a maskinglayer on at least a portion of the cavity surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] For a more complete understanding of the present invention, andthe advantages thereof, reference is now made to the followingdescriptions taken in conjunction with the accompanying drawings, inwhich:

[0011]FIG. 1 is a perspective view of a small portion of a micromirrorarray of the prior art.

[0012]FIG. 2 is an exploded perspective view of a single micromirrorelement from the micromirror array of FIG. 1.

[0013]FIG. 3 is a cross section side view of a micromechanical device ina package according to one embodiment of the present invention.

[0014]FIG. 4 is a perspective view of a portion of a ceramic substrateaccording to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention provides an improved package and packagesubstrate that is useful for packaging of MEMS devices. Particlegeneration by the substrate is reduced by the application of a linerover the inner ceramic surfaces. The liner is particularly useful whenapplied over the rough edges of the substrate cavity wall and over thesharp edges of the substrate.

[0016] The improved package substrate of the present invention isparticularly useful when used to package micromirror devices such as theone shown in FIGS. 1 and 2. As shown in FIG. 1, a typical hidden-hingemicromirror 100 is actually an orthogonal array of micromirror cells, orelements. This array often includes more than a thousand rows andcolumns of micromirrors. FIG. 1 shows a small portion of a micromirrorarray of the prior art with several mirrors 102 removed to show theunderlying mechanical structure of the micromirror array. FIG. 2 is anexploded view of a single micromirror element of the prior art furtherdetailing the relationships between the micromirror structures.

[0017] A micromirror is fabricated on a semiconductor, typicallysilicon, substrate 104. Electrical control circuitry is typicallyfabricated in or on the surface of the semiconductor substrate 104 usingstandard integrated circuit process flows. This circuitry typicallyincludes, but is not limited to, a memory cell associated with, andtypically underlying, each mirror 102 and digital logic circuits tocontrol the transfer of the digital image data to the underlying memorycells. Voltage driver circuits to drive bias and reset signals to themirror superstructure may also be fabricated on the micromirrorsubstrate, or may be external to the micromirror. Image processing andformatting logic is also formed in the substrate 104 of some designs.

[0018] The silicon substrate 104 and any necessary metal interconnectionlayers are isolated from the micromirror superstructure by an insulatinglayer 106 which is typically a deposited silicon dioxide layer on whichthe micromirror superstructure is formed. Holes, or vias, are opened inthe oxide layer to allow electrical connection of the micromirrorsuperstructure with the electronic circuitry formed in the substrate104.

[0019] Address electrodes 110 and a mirror bias connection 112 areformed from a metal layer deposited on the insulating layer 106. Somemicromirror designs have separate and distinct landing electrodes thatare electrically connected to the mirror bias connection 112. Landingelectrodes limit the rotation of the mirror 102 and prevent the rotatedmirror 102 or hinge yoke 114 from touching the address electrodes 110,which have a voltage potential relative to the mirror 102. If the mirror102 contacts the address electrodes 110, the resulting short circuitcould fuse the torsion hinges 120 or weld the mirror 102 to the addresselectrodes 110, in either case ruining the micromirror. Since the samevoltage is always applied both to the landing electrodes and the mirrors102, the mirror bias connection and the landing electrodes are combinedin a single structure when possible. The landing electrodes are combinedwith the mirror bias connection 112 by including regions on the mirrorbias/reset connection 112, called landing sites, which mechanicallylimit the rotation of the mirror 102 by contacting either the mirror 102or the torsion hinge yoke 114. These landing sites are often coated witha material chosen to reduce the tendency of the mirror 102 and torsionhinge yoke 114 to stick to the landing site.

[0020] Hinge support spacervias 116 and upper address electrodespacervias 118, typically extend approximately 1 μm above the addresselectrodes 110 and mirror bias connections 112. A hinge cap 122 andupper address electrodes 124 are supported by the hinge supportspacervias 116 and upper address electrode spacervias 118. The hinge cap122 anchors the ends of torsion hinges 120. A hinge yoke 114 is formedbetween and supported by the torsion hinges 120. The hinge yoke 114 isallowed to rotate by twisting the thin torsion hinges 120. A mirrorsupport spacervia 126 is formed on the hinge yoke, and supports a mirror102 approximately 2 μm above the hinge yoke 114.

[0021] Electrostatic attraction between an address electrode 110 and adeflectable rigid member, which in effect form the two plates of an airgap capacitor, is used to rotate the mirror structure. Depending on thedesign of the micromirror device, the deflectable rigid member is thetorsion beam yoke 114, the beam or mirror 102, a beam attached directlyto the torsion hinges, or a combination thereof. The upper addresselectrodes 124 also electrostatically attract the deflectable rigidmember.

[0022]FIG. 3 is a cross section side view of a micromechanical device,in a package according to one embodiment of the present invention. Themechanical device 302, for example a micromirror device, is attached tothe floor 304 of the package substrate 306, typically by epoxy 308.Electrical connections are provided by bond wires 310 connecting bondpads 312 on the substrate with bond pads on the device 302. A window 314is attached to the substrate 306 to form a cavity 316 around the device.

[0023] Ceramic is the material of choice for hermetic device packagesubstrates. Unfortunately, ceramic substrates create many particles thatcan destroy the MEMS device. Of particular concern are the sharp edgesand cavity walls. The sharp edges create particles because theconcentration of forces on the thin sharp edge makes it easy forparticles to break away from the edge. The exterior edges of the ceramicsubstrate are chamfered to prevent this. Unfortunately, there is noautomated process for chamfering the inner edges of the cavity.

[0024] A second particle-rich area is the rough cavity wall. Not onlydoes the rough surface of the inner cavity wall harbor particles, therough surface creates many points susceptible, like the sharp edgesmentioned above, to fracture. Furthermore, the rough edges are oftenfractured during substrate manufacture leaving particles that can easilybreak free. Current processes use a chemical process to smooth the innercavity wall. This is a manual process and is expensive and timeconsuming to perform. The surfaces of the package cavity not only aredifficult to clean, and therefore likely to harbor and create particles,but any particles created by the cavity surfaces will be captured in thesealed package where they are likely eventually to damage the packageddevice.

[0025] A solution to the particle generation problem is to apply a thinmasking layer 318 to the portions of the package cavity surface thatsource particles. The thin masking layer may be any material thatresists particle generation. Malleable metals are ideal. For example,gold plating on the inner cavity walls bonds well to the ceramicsurfaces, captures particles that are already loose, and resistschipping to avoid the generation of additional particles. A furtherbenefit of gold plating is the ability of gold, as well as many othermetals, to withstand the lubricants often used in MEMS packages. Forexample, the perfluordecanoic acid used in micromirror packages. Sincegold is already used to form the bond pads 312 on the cavity floor, theadditional steps need to add the plating to the cavity walls areminimal.

[0026] As shown in FIG. 3, the masking layer 318 on the cavity wallsextends out of the cavity and onto the upper surface of the package.Although this extension 320 is not necessary, it does make it easier tocompletely enclose the upper sharp edge of the cavity. The extension 320may also be used to attach the window 314, in the case of a micromirrorpackage, or lid. A further benefit of the extension 320 of the maskinglayer 318 is that it raises the window 314 further above the MEMS device302. In a micromirror system, raising the package window moves itfurther out of the focal plane and makes defects in the glass lessvisible—allowing higher yields with the use of cheaper materials.

[0027]FIG. 4 is a perspective view of a portion of a ceramic substrateaccording to one embodiment of the present invention. In FIG. 4, themasking layer 318 extends across the floor 304 of the package cavity.Thus, the masking layer nearly covers the entire substrate in thecavity. When a conductive masking layer 318 is used, for example a metallayer, gaps must be formed between the bond pads 312 and the maskinglayer 318.

[0028] The masking layer described above may be applied in a number ofways. For example, a metal masking layer may be sputtered over thecavity surface, with areas masked to prevent bonding with the substrate.The masking layer may also be plated on the surface. Other maskingmaterials, such as epoxy, lend themselves to methods of spraying orbrushing the masking layer onto the surface of the cavity. Excessmasking materials may be removed by chemical processes, such as variousetch steps, or by laser etching.

[0029] Thus, although there has been disclosed to this point aparticular embodiment for a masking layer in a substrate cavity andmethod therefore, it is not intended that such specific references beconsidered as limitations upon the scope of this invention exceptinsofar as set forth in the following claims. Furthermore, havingdescribed the invention in connection with certain specific embodimentsthereof, it is to be understood that further modifications may nowsuggest themselves to those skilled in the art, it is intended to coverall such modifications as fall within the scope of the appended claims.In the following claims, only elements denoted by the words “means for”are intended to be interpreted as means plus function claims under 35U.S.C. §112, paragraph six.

What is claimed is:
 1. A method of forming a package substratecomprising: forming a substrate, said substrate having a cavity; andcoating portions of said cavity surface with a masking layer.
 2. Themethod of claim 1, said forming comprising forming a ceramic substrate.3. The method of claim 1, said coating comprising coating walls of saidcavity with said masking layer.
 4. The method of claim 1, said coatingcomprising coating walls of said cavity with a gold masking layer. 5.The method of claim 1, said coating comprising coating walls of saidcavity with an epoxy masking layer.
 6. The method of claim 1, saidcoating comprising sputtering walls of said cavity with a metal layer.7. The method of claim 1, said coating comprising plating walls of saidcavity with a metal layer.
 8. The method of claim 1, said coatingcomprising coating walls of said cavity with a masking layer extendingonto an upper surface of said substrate.
 9. A package comprising: asubstrate, said substrate having an upper surface and a cavity surfaceformed therein, said cavity having at least one wall surface and atleast one floor surface; a device attached to said cavity floor; a lidattached to said upper surface enclosing said device; and a maskinglayer on at least a portion of said cavity surface.
 10. The package ofclaim 10, said lid comprising a window.
 11. The package of claim 10,said masking layer comprising a metal masking layer.
 12. The package ofclaim 10, said masking layer comprising an epoxy masking layer.
 13. Thepackage of claim 10, said masking layer comprising a gold masking layer.14. The package of claim 10, said masking layer comprising a maskinglayer on said cavity walls.
 15. The package of claim 10, said maskinglayer comprising a masking layer on said cavity floor.
 16. The packageof claim 10, said masking layer comprising a masking layer extendingonto said upper surface.
 17. The package of claim 16, said lid attachedto said masking layer extending onto said upper surface.
 18. A packagesubstrate comprising: a substrate, said substrate having an uppersurface and a cavity surface formed therein, said cavity having at leastone wall surface and at least one floor surface; and a masking layer onat least a portion of said cavity surface.
 19. The package substrate ofclaim 18, said masking layer comprising a metal masking layer.
 20. Thepackage substrate of claim 18, said masking layer comprising an epoxymasking layer.
 21. The package substrate of claim 18, said masking layercomprising a gold masking layer.
 22. The package substrate of claim 18,said masking layer comprising a masking layer on said cavity walls. 23.The package substrate of claim 18, said masking layer comprising amasking layer on said cavity floor.
 24. The package substrate of claim18, said masking layer comprising a masking layer extending onto saidupper surface.